Which quantity is defined as the number of protons in an atom?

Which quantity is defined as the number of protons in an atom?

• A. Mass Number
• B. Atomic Number
• C. Number of Protons
• D. Charge

Answer: (B)

The atomic number is the number of protons in an atom. This quantity uniquely identifies the element because each element has a characteristic count of protons, and all atoms of that element share it. The mass number, in contrast, is the sum of protons and neutrons, so it varies among isotopes of the same element. Saying “number of protons” describes the same count, but the standard term used is atomic number. Charge expresses the net electric charge, which depends on the balance of protons and electrons; a neutral atom has zero charge, while ions carry positive or negative charge.

The atomic number is the number of protons in an atom. This quantity uniquely identifies the element because each element has a characteristic count of protons, and all atoms of that element share it. The mass number, in contrast, is the sum of protons and neutrons, so it varies among isotopes of the same element. Saying “number of protons” describes the same count, but the standard term used is atomic number. Charge expresses the net electric charge, which depends on the balance of protons and electrons; a neutral atom has zero charge, while ions carry positive or negative charge.